What Cp and Cpk really tell you — and what they don’t.
18 September 2026 · 1 min read
Capability indices summarise how well a process fits within its specification limits. They are useful, but only when their assumptions hold.
Check stability first
A capability index computed on an unstable process describes nothing in particular. Confirm statistical control before calculating.
Look at the data, not just the number
Two processes with the same Cpk can behave very differently. Always plot the distribution.
Illustrative sample article.